1. Field of the Invention
The present invention relates to an electron-beam exposure apparatus component and in particular, to a cell projection aperture formation method.
2. Description of the Related Art
Semiconductor electronic parts such as a dynamic random access memory (DRAM) have been increasing their integration degree, requiring an ultra-fine processing technique. As means for implementing an ultra-fine resist pattern in a semiconductor production process, a great expectation is posed on an electron-beam exposure method employing a partial cell projection method. This exposure method uses a cell projection aperture of a Si substrate having an aperture of configuration identical to a part of a semiconductor pattern or the entire semiconductor pattern.
Conventionally, a cell projection aperture has been prepared by employing a conventional semiconductor processing technique such as resist patterning using photolithography, the etching technique using plasma, or the wet etching technique, so as to obtain a desired pattern through a Si substrate. (See Y. Nakayama, H. Satoh, et al "Thermal Characteristics of Si Mask for EB Cell Projection Lithography", Jpn. J. Applied Physics, Vol. 31 (1992), pp. 4268 to 4272, Part 1, No. 12B, December 1992 [1], and in Y. Nakayama, S. Okazaki, and N. Satoh "Electron-Beam Cell Projection Lithography: A New High-Throughput Electron Beam Direct-Writing Technology Using a Specially Tailored Si Aperture", J. Vac. Sci. Technology B8 (6), Nov/Dec 1990 [2])
More specifically, a Si substrate is firstly subjected to a plasma etching to the depth of 15 to 30 micrometers from the surface. After this, the Si substrate is subjected to a back etching so as to open a pattern from the back of the Si substrate by etching the remaining 500 to 600 micrometers.
FIG. 7 shows a cell projection aperture formation method according to the aforementioned documents [1] and [2]. Hereinafter, explanation will be given on the cell projection aperture formation method according to respective cell projection aperture formation steps.
Firstly, the Si substrate surface is spin-coated with resist 5 with a thickness of 1 micrometer, which is subjected to exposure using a g-ray or i-ray demagnification projection exposure apparatus, contact exposure apparatus, electron-beam exposure apparatus or the like, and developed to obtain a resist pattern (FIG. 7A Cell pattern exposure/development step). The Si substrate often used is a double-sided Si substrate 8 (hereinafter, referred to simply as Si substrate 8) consisting of two Si substrates 8a and 8b bonded together via a Si oxide film as an etching mask 9.
Next, using this resist 5 as an etching mask, the Si substrate 8 is plasma etched in the depth of about 20 micrometers (FIG. 7B Si etching step). Here, in order to obtain a preferable etching configuration, a Si oxide film can be used instead of the resist 5 as the etching mask. In this case, prior to the resist application, a Si oxide film is formed on the surface of the Si substrate with a thickness of about 1 micrometer using a thermal oxidation or CVD (chemical vapor deposition), after which the Si oxide film is subjected to etching through the aforementioned resist pattern for patterning of the Si oxide film. Next, the resist 5 is peeled off so that the patterned Si oxide film is used as an etching mask for etching the Si substrate 8.
Next, a nitride film 6 is formed on the top surface and the back surface of the Si substrate 8, and furthermore a resist 5 is applied. Alignment is performed with respect to the top surface pattern, and the back surface pattern is exposed and developed for patterning the resist 5 (FIG. 7C, nitride film formation, resist application/exposure/development step).
This resist pattern is used as an etching mask for etching the Si nitride film 6. After the resist is removed, using the Si nitride film 6 as an etching mask, the Si substrate 8 is subjected to 600-micrometer etching from the back surface using a KOH solution or the like. (FIG. 7D, back etching step).
Finally, the Si nitride film 6 is removed using thermal phosphoric acid, and the top surface of the Si substrate 8 is covered with a metal film such as Au as a conductive layer 7 (FIG. 7E, conductive layer formation step). The substrate is cut into a predetermined size, thus completing a cell projection aperture.
As has been described above, the conventional production method is rather complicated and requires an expensive semiconductor production apparatus and a high processing technology. There is also a problem that the yield is too low.
An ordinary demagnification ratio of a cell projection radiation type electron-beam exposure apparatus, i.e., the ratio of the dimension on a wafer against the aperture dimension is 1/10 to 1/100. For example, in order to obtain a 0.1-micrometer pattern on a wafer, the pattern has a width of 1 to 10-micrometers in the aperture. If a Si is to be etched to the depth of 15 to 20 micrometers for this width, the aspect ratio (ratio of depth to width) becomes 20 at maximum. Currently, this is technically difficult to obtain.
Moreover, the back etching of the Si substrate to 500 to 600 micrometers from the back surface using an etching solution such as KOH requires a long period of time because the etching rate is low. For this, the surface of the Si substrate is deteriorated by the etching solution during the etching, resulting in a low yield.
Moreover, the semiconductor device production requires a processing dimensional accuracy of 5% or below. Accordingly, the aperture processing accuracy should have a high accuracy of 0.05 to 0.5 micrometers. Furthermore, in order to allow to pass an electron-beam with a preferable configuration, the taper angle after an etching should be 89 degrees or above. Such a strict processing accuracy is difficult to obtain by the conventional technique.
Furthermore, with the conventional technique, it is difficult to form apertures having different depths. Accordingly, it is difficult to produce a cell projection aperture in which the proximity effect is compensated by differing the electron-beam transmittance at desired positions of a pattern.